step one: don't fucking kill the pet.step two: get ready for litigation if it's a cat/dog/bird/other "fuzzy wuzzy" animal. also be prepared to lose your friend.step three: you're probably going to owe them a lot of alcohol if they decide you're not enough of a douche canoe to fade on.

it was my sister's "unkillable feeder goldfish," which is really just a common goldfish that survived terrible care.

i killed a friend's terrapin (my mom's tap water is hella arsenic-y) and he told me that with terrapins if they die, the story is that it basically took an arrow for you, you were gonna have something bad happen, so just be glad it happened to the terrapin not me, and was cool about it. in general... apologize a shit-ton and offer to buy a new one??